The pulse width modulator 300, depicted in FIG. 2, is connected to a circuit system 8. The circuit system 8 comprises at least three sets of upper and lower field effect transistors (fets) and a sense resistor. The modulator 300 commutates (switches) the fets of the system 8. Modulator 300 comprises a peak current detector 310, a one shot device 320, a decoder 330, a commutation detect circuit 340, and a commutation state machine 350.
The peak current detector 310 receives a current signal 28 indicating the total current in system 8. Peak current detector 310 compares the current signal 28 to a predetermined reference signal R. The peak current detector 310 generates a detector signal 312. The detector signal 312 is either a nonpeak signal or a peak signal. The peak signal is generated when the current signal 28 is a predetermined ratio of the reference signal R.
The one shot device 320 receives the detector signal 312. The one shot device is a multivibrator that generates a pulse output 322. When the one shot device 320 receives the peak signal the device 320 generates a pulse 322. The pulse is about 1 microsecond long.
The decoder 330 comprises a PWM input that receives pulse 322 and 3 H inputs (H1, H2 and H3) that control the commutation of the fets. When decoder 330 receives the pulse 322 PWM is set to high. When PWM is high, the current in the circuit system 8 ramps down. The time during ramp down is known as off-time because the gate of the pulse width modulating fet is off. Once the pulse 322 concludes PWM reverts to low and the current in the circuit system ramps up (otherwise known as on-time) because the pulse width modulating fet is on.
Commutation is defined as routing current from one phase to another phase. An upper commutation involves the upper fets in 2 phases and a lower commutation involves the lower fets in 2 phases. During commutation the current cannot pass through both fets of the same circuit. A three phase system has six states of commutation for the six fets.
Commutation occurs when commutation detector 340 generates a commutation pulse 342. The commutation state machine 350, electrically connected to the decoder 330 and the commutation detector 340, receives the commutation pulse 342 and then initiates commutation by generating a signal 356 to decoder 330. The machine 350, in conjunction with H1, H2, H3, commutates the fets in revolution. A revolution is when each of the six states of commutation occurs before repeating one of the six states. During commutation the one shot device 320 generates the one microsecond pulse during the off time of the fets.
Commutation with the prior art pulse width modulator on a brushless dc motor driver system has a dip in the total current of the system. A dip in current occurs because the current in the load winding being commutated (switched off) drops faster than the current rising in the load winding that is commutating (switched on). Various pulse width modulators have been used in the past to control that dip with little success.
When the total current dips, a torque ripple can result in mechanical vibrations that are audible. In a typical motor such as one in a computer disk drive, the noise of the vibration is not only annoying, but may also corrupt data written to the disk or lead to premature failure of the disk drive.
An object of the present invention is to prevent that dip in the current of a load driver system during commutation. A further objective is to provide a quieter disk drive with a longer life and more accurate data storage.